Graph in blender a function of two variables,
There is an addon shipped by default with Blender called Add Mesh: Extra Objects capable of doing this. It is disabled by default and can be activated from the User Preferences dialog.

Start by opening File > User Preferences > Addons then search for Extra and activate the Add Mesh: Extra Objects addon.

Then in the 3D View press Shift+A > Add > Mesh > Math Function > Z Math Function or XYZ Math Function depending on what you want.

Introduce the function equation in the corresponding fields and adjust precision as necessary.

See related question for more details.